It is well known in the art to convert olefins to aldehydes having one additional carbon atom by contacting the olefin with hydrogen and carbon monoxide in the presence of a catalyst based on cobalt or rhodium metal. Rhodium-based catalysts have the advantage, relative to cobalt-based catalysts, of being able to promote the hydroformylation of olefins under less severe operating conditions.
One disadvantage of prior art rhodium-based catalysts is the propensity of such materials to lose activity over a period of time as a result, for example, of ligand decomposition. Triaryl phosphines, for example, are prone to conversion into alkyl diaryl phosphines under hydroformylation reaction conditions. These alkyl diaryl phosphines as rhodium ligands give lower activity catalysts compared to the triaryl phosphines.
Another disadvantage of prior art rhodium-based catalysts is the fact that not all rhodium salts are suitable starting materials for the preparation of rhodium complexes. For example, it is frequently observed that a several hour induction period is required to transform the rhodium complexes into active hydroformylation catalysts. This problem is particularly acute when halide containing compounds of rhodium are employed for the preparation of rhodium complexes.
Yet another disadvantage of rhodium-based catalyst systems is the high cost of the rhodium metal employed for catalyst preparation. Where one employs low levels of rhodium metal in order to reduce catalyst costs, low reaction rates frequently result.
There is, therefore, a continuing need in the field for high activity, high selectivity rhodium-based hydroformylation catalyst systems. Especially desirable would be high activity catalyst systems which can be controlled to produce a wide range of product ratios of normal/branched chain aldehyde products. Prior art rhodium-based hydroformylation catalyst systems typically lack such flexibility as they are only capable of producing very narrow normal/branched chain product ratios while at the same time retaining high catalyst activity. Efforts to vary the normal/branched chain product ratio significantly from that which a particular catalyst can inherently produce typically results in dramatically reduced catalyst activity.